Nucleating agents may improve polymer properties in several ways. When used in polymers, such as polypropylene, nucleating agents may increase the rate of crystallization. Such nucleating agents may change the polymer crystallization temperature characteristics. This may provide benefits in polymer manufacturing processes, as well as the final aesthetics of polymer articles.
Injection molding polymer applications frequently use nucleating agents. Blow molding, sheet extrusion, and thermoforming applications also may benefit from their use. Nucleating agents may reduce cycle time by reducing the set-up time in the mold. However, care must be taken to ensure that shrinkage and impact properties are not negatively impacted.
The optical benefits of nucleating agents include increased clarity and improved gloss in finished polymeric articles. These properties may improve because of the dramatic increase in the number of fine crystals. When crystals are smaller than the wavelength of visible light, the light passing through the article is much less scattered, thereby haze is reduced when nucleating agents are used. When utilized to improve transparency in materials such as polypropylene, these materials are referred to as clarifiers, or clarifying agents.
Five major categories of nucleating agents include: substituted sorbitols, carboxylic acid salts such as sodium benzoate, low molecular weight polyolefins, ionomer resins, and organophosphate salts.
One nucleating agent marketed by Milliken & Company is known as HYPERFORM™, or HPN-68™, as shown in U.S. Pat. No. 6,465,551. This product, which comprises a dicarboxylate salt, is commonly known as “hyper” nucleating agent. It is commonly used in injection molded polypropylene. However, this product is not used widely as a clarifier in most instances, in part because a lack of adequate dispersion in the polymer does not provide sufficient optical clarity in finished articles.
Another nucleating agent marketed by Asahi Denka Corporation of Japan is NA-21™. The compound of this nucleating agent is disclosed in U.S. Pat. No. 5,342,868. This compound is mostly used as a clarifier, and does not provide adequate nucleation benefits to serve as a true nucleating agent for most polymer applications.
It is a significant challenge in the industry to locate compounds that are capable of affording both good nucleating ability and good optical clarity benefits in a polymer. There is a substantial need for agents that provide both nucleation benefits and clarity benefits when employed in polymer systems. This invention addresses that need.